High resolution NMR studies are characterized by disposition of a sample on the axis of, and surrounded by the RF coil of the NMR probe at an axial position of excellent homogeneity of the polarizing field. A critical aspect of the apparatus is its sensitivity, which is a function of the properties of the RF coil, the temperature, and the geometric relationship of the sample to the interior space of the coil (filling factor). A quantitatively high signal amplitude requires a corresponding large quantity of sample, and thus a careful geometric match of sample to the RF coil, yielding a high filling factor, is desired. The filling factor is limited by the available transverse interior dimension of the RF coil. The aperture (RF window) of the coil determines the effective axial dimension of sample volume, but the physical volume of sample customarily extends beyond the coil aperture in accord with standard practice. Some transverse dimension is inevitably lost to the wall thickness of the sample container and any clearance between sample container and the RF coil, with the result that the volume of sample presented for study is always less than the (available) interior volume of the RF coil. So long as other factors effecting sensitivity have been optimized, maximum sensitivity is reached when the filling factor is maximized. Practical constraints limit maximum filling factor, optimum conditions for NMR studies are understood to include such practically achievable limits.
The modern NMR probe can accommodate a plurality of coaxial RF coils and each coil is a component of a resonant circuit which is tunable over a range of frequencies and adjustable in impedance match to the corresponding RF source/sink. A probe may further contain one or more preamplifier modules to condition received signals. Ambient temperature control of the sample and temperature monitoring components are typical features. Controlled high speed rotation of the sample container requires pressurized gas control for levitation of a sample turbine on air bearings, together with a separate pressurized gas control for turbine rotation and rotational rate detector. The RF coil (of either superconducting or normal conductor) may utilize temperature control. Consequently, an NMR probe is an expensive and complex instrument.
Although it is desirable for the quantity of sample to be “sufficient” for high quality NMR data, such sufficiency is a geometric parameter of the probe design. NMR practitioners have adopted certain standard sample container dimensions which accommodate typical conditions. For example, it is currently common for NMR studies to employ 5 mm o.d. sample tubes. Throughout this work the term “macro mode” refers to data acquisition from sample presented in such volume as available with 5 mm sample tubes (or the equivalent) in combination with an RF resonator exhibiting optimal filling factor for that sample. It may also be the case in some studies that some samples are characterized by unusually limited availability and it is known practice to utilize “micro coils” for these studies. Apart from the sample availability, enhanced sensitivity is achievable with micro samples closely matched to a micro-coil. The term “micro coil” is meant to convey a dimensional scale that is significantly smaller than the dimensions accommodating the standard sample. The contemporary standard analytic sample is presented to the probe in a 5 mm o.d. pyrex or quartz tube. The micro-coil is most often, a component of a purpose-built probe. It is conventional to present sample for “micro mode” studies in 3 mm. o.d. sample tubes where the micro-coil exhibits a high filling factor to such samples. For the purposes of this work, “micro mode” operations may be so defined in relation to macro studies. Redundancy in expensive probe apparatus to implement similar NMR studies of both plentiful sample and micro-samples is an expense that this work seeks to ameliorate.
The use of small volume samples in appropriately scaled NMR probes is reviewed at volume 56, Annual Reports on NMR Spectroscopy, pp. 2-88 (Academic Press, 2005) where the enhanced sensitivity for 3 mm. NMR studies is reviewed.
In prior art, the use of micro coils is well known and summarized in U.S. Pat. Nos. 5,654,636 and 6,097,188. It is also known to use a micro coil supported on a sample tube as a self-resonant circuit inductively coupled to the fixed RF coil of the NMR probe. See WO2007/020537. Dual use of the same probe for a variety of sample availabilities and requirements for the probe to obtain this benefit are not disclosed therein.
Inductively coupling in probe structure is well known. Representative of the art are: Kuhns, et al, J. Mag. Res., vol. 78, pp. 69-76 (1988); Schnall, et al, J. Mag. Res. Vol. 68, 161-167 (1986); and from the review by Mispelter, et al., NMR Probeheads for Biophysical and Biomedical Experiments, especially chapt. 3, (Imperial College Press: London, 2006).